In a device for measuring rotational angles of a motor, the rotation of the motor shaft must be mechanically extracted to the device side. There are some cases in which the rotation cannot be extracted from a shaft coaxially coupled to the motor shaft via a coupling such as when the motor shaft is a hollow shaft having a hollow part in which another parts or cables are arranged.
In such cases, a gear is attached coaxially to the motor shaft and the rotation of the motor shaft is extracted from a transmission gear disposed to mesh with the gear, whereby the rotation of the motor shaft can be extracted to a position which is radially and outwardly apart from the rotation center of the motor shaft. When a small-diameter gear as the transmission gear for extracting rotation is disposed on an outward and radial side of the hollow motor shaft, the motor rotation is extracted in a state in which the rotational speed is increased. For example, in a case of a servomotor rotating at a rate of several thousand rotations per minute, a small-diameter transmission gear is rotated at a rate of several tens of thousand rotations per minute, which is not practical.
It is known as a mechanism for extracting the rotation of a rotating shaft to make use of magnetic coupling. Patent document 1 proposes an encoder device configured to extract the rotation of a motor shaft by making use of magnetic gears. Patent document 2 proposes a mechanism to extract rotation by making use of a magnetic type worm gear. In either case, a magnetic gear is provided, which is configured to be so magnetized as to correspond to each tooth of a spur gear or a worm gear.